1. Field of the Invention
When phosphate rock with a relatively high organic content, as for example, rock from North Carolina and the western United States, is utilized in the manufacture of wet-process phosphoric acid, such rock is preferably first thermally treated to remove therefrom such organic matter. In practicing the wet-process for effecting the manufacture of phosphoric acid, such resulting thermally treated rock is subsequently reacted in an acidulation step with a mixture of phosphoric and sulfuric acids followed by filtration of the resulting calcium sulfate formed in said acidulation step to produce a filtrate of phosphoric acid. It has long been known that residual organic matter in such phosphate rock may cause severe foaming and filtration problems during such acid manufacture. Sulfides present in such calcined rock are also known to cause filtration problems and said sulfides have for some time been suspect of causing marked increases in equipment corrosion when effecting the processing of such rock. A particularly objectionable characteristic of such high organic content rock is the attendant formation, during the calcination thereof, of what shall, for the sake of convenience, be hereinafter referred to and termed acid-evolved sulfide; i.e., the sulfide which is evolved as a noxious gas, such as hydrogen sulfide, during acidulation of the rock, which gas is known to be both a health and an environmental hazard. Acid-evolved sulfide can originate from many sources, usually after reaction of the source material during thermal treatment of the rock. Sources thereof include sulfur contained in gangue minerals, such as gypsum or pyrite, sulfur exsolved from the apatite crystal lattice during heating of the rock, elemental sulfur impurities, the generally high sulfur content of organic matter associated with phosphate rock, and sulfur derived from the fuel used in said thermal treatment.
Calcining of, for example, North Carolina rock, as practiced commercially at temperatures usually greater than 800.degree. C., can virtually eliminate organic matter and sulfide formation, but such prior art practice unfortunately results in a calcined product exhibiting undesirably low effective surface areas (usually less than 0.4 m.sup.2 /g) resulting in turn in rather poor reactivity of the rock when mixed with acid, as for example, when subsequently utilized in the manufacture of phosphoric acid or superphosphate.
It has been suggested that to qualify as a truly improved thermal or calcination process for affecting rock of the type characterized herein, levels of acid-evolved sulfide and residual organic matter in the calcined rock product effected thereby should be minimized and at the same time the surface area of such resulting calcined phosphate rock intermediate product should advantageously be maintained at levels of greater than about 2 m.sup.2 /g to ensure that same will exhibit improved and sufficient reactivity to acid attack in the subsequent manufacture of wet-process phosphoric acid.
It is to be herein emphasized that phosphate rock of the class generally known as francolites may be drawn from different rock deposits, and whether from the same deposit or from different deposits, may be found to contain a variety of organic matter concentrations, impurity mineral inclusions and substitutions in the francolite crystal lattice, and that these factors can contribute to a wide range of behavior patterns upon the thermal treatment thereof. I have found, in particular, that North Carolina phosphate rock and its contained organic matter is extremely sensitive to methods of thermal treatment and that application of prior art methods established with phosphate rock types from other deposits does not necessarily result in a satisfactory calcined product from North Carolina rock. Due to the high degree of carbonate substitution in the francolite lattice, use of high calcination temperatures on North Carolina rock is particularly undesirable; as referred to supra temperatures above 800.degree. C. cause dramatic loss of surface area and consequently undesirably decreased reactivity during subsequent acidulation processes. Similarly, use of temperatures in the range of 700.degree. C. to 800.degree. C. can cause undesirable buildup of deleterious acid-evolved sulfide in the calcined product. In contrast, removal of organic carbon from such North Carolina rock is usually difficult in that the organic matter in the rock has a great propensity for formation of intractable char, which char can only be removed in a calcination process by heating to rather high temperatures.
In view of these conflicting heating requirements for producing, from rock of the type described, a desirable calcined intermediate product having both low organic and acid-evolved sulfide contents but with retained high surface area, the present invention has evolved as a new, novel, and energy-efficient method for thermally processing a "difficult" phosphate rock such as that from North Carolina.